Alcohol binge drinking is associated with dangerous levels of intoxication that are costly to both the individual and society. NIAAA (2004) and others have defined a binge as a drinking episode that produces a blood alcohol concentration of e .08 percent, which typically corresponds to five drinks for men and four drinks for women consumed within 2 hours. Most of what is known about binge drinking is based on studies of college students, however, most binge drinkers are older adults. One mechanism that may play an important role in alcohol binging is a loss of control (or increased impulsivity) occurring after the initial consumption of alcohol. Recent evidence suggests that individuals with reduced serotonin function may experience disproportionate increases in impulsivity after consuming alcohol. The proposed study of binge and non-binge drinkers will determine how impulsivity is affected by alcohol and/or reductions in serotonin synthesis, and then to determine how these observed effects are related to the capacity to modify drinking patterns. Adult Binge (n = 224) and Non-Binge (n = 56) drinkers between the ages of 26 and 54 will be recruited from the community and participate in four study phases. In Phase I, participants complete a battery of different impulsivity measures during a simulated alcohol binge procedure, after a procedure altering L-tryptophan availability (affecting serotonin synthesis). Conditions include: (1) alcohol consumed after L-tryptophan depletion; (2) placebo consumed after L- tryptophan depletion; (3) alcohol consumed after L-tryptophan balanced control; and (4) placebo consumed after L-tryptophan balanced control. In Phase II, drinking patterns among the two groups are characterized using continuous transdermal alcohol monitoring (and self-report) across a 4-week period. In Phase 3, Binge drinkers complete a 12-week contingency management procedure designed to reduce alcohol consumption and produce a range of drinking outcomes. Lastly, in Phase 4, participants experience a brief motivational interview and patterns of drinking are re-evaluated monthly for 3 months. This study allows us to address three primary aims: to characterize behavioral impulsivity among adult Binge and Non-Binge drinkers, as well as their responses during a simulated alcohol binge and/or L-tryptophan depletion challenges (Aim 1); to use a 12-week contingency management procedure to determine to what extent individual differences in the capacity to reduce drinking among alcohol bingers is related to the behavioral impulsivity responses observed during the alcohol and/or L-tryptophan depletion challenge procedures (Aim 2); and to determine to what extent reductions in drinking achieved during contingency management can be maintained across a 3-month follow- up period that begins with a brief motivational interview, and to determine whether behavioral responses observed in Specific Aim 1 predict observed outcomes (Aim 3). This study represents a critically important step by relating biological and behavioral markers to outcomes observed in the real world and will lay the groundwork necessary for future studies.